To understand the causes and consequences of health care intensity, it is crucial to have a comprehensive database with sufficient power to characterize detailed patterns of health care utilization at the region or even hospital level. The Medicare claims data are ideal for this task because of three key features. First, they are national in scope: 98% of the elderly are eligible for Medicare, and because almost all hospitals and physicians provide care to Medicare enrollees, the data can provide insight into factors that influence both hospital performance and physician practice more generally. Second, the claims data provides detailed demographic and financial information, as well as rich clinical detail on the specific services provided to fee-for-service enrollees. Third, with appropriate approvals to preserve confidentiality, the data can be obtained with individual identifiers that can support linkage across files and over time. The utility of these data for a wide range of health care and economic research studies has been amply demonstrated. The complexity and costs of analyzing these data motivate the specific alms of this Data Core: 1. To process, match, and integrate the year 1999-2004 Medicare claims data. To support the analytic tasks of the four research projects, this Data Core will provide the computing and programming infrastructure necessary to prepare analytic files from the Medicare administrative databases, provide access to previously obtained Medicare claims files, and will obtain and prepare for analysis additional years of Medicare claims and research databases. 2. To make the survey and Medicare data available to the research community. The survey and Medicare claims data would be compiled and made available in restricted form. This would allow for a wide range of research uses, but it would not present the same security concerns as the raw Medicare claims data. In addition, we would plan to provide training in the use of the Medicare claims data.